1. Field of the Invention
This invention generally relates to solar generated power and, more particularly, to a hybrid system that combines thermal and asymmetrical photovoltaic concentration for energy generation and thermal storage.
2. Description of the Related Art
There are two basic approaches to the harvesting of solar energy using concentrating optics. These approaches are: concentrating solar power (CSP), which uses a thermal collector and thermal engine, and concentrating photovoltiacs (CPV), which concentrates sunlight onto high-efficiency multi-junction PV cells for direct electrical generation. CSP can be configured to include thermal storage, making its output dispatchable, while CPV is known for its very high efficiency. Therefore, the optimal approach is a system that combines CSP with additional tracking CPV optics.
FIG. 1 is a solar energy collection device using a parabolic trough (prior art). The design of Brunotte et al. [1] is one of the earliest to attempt to convert a line focus into a series of higher concentration foci. In this design, a parabolic trough is used to illuminate a series of solid compound parabolic concentration (CPC) secondaries, each conjugate to a single photovoltaic (PV) cell. The trough is tracked about a polar-aligned axis, so the range of skew angles of sunlight is limited to +/−23.5° throughout the year. The CPCs have square or rectangular apertures, and may have an asymmetric acceptance angle. The acceptance angle in the transverse plane of the trough is determined by the maximum rim angle of the trough. If an asymmetric trough is used, as shown, then the CPCs can be tilted to a median angle, and the acceptance angle is only half the rim angle of the trough. In the other dimension (along the axial plane of the trough), the CPCs are required to have an acceptance angle of +/−23.5°, to maintain seasonal performance.
FIGS. 2A and 2B depict a design of tracking secondaries for conventional troughs, off-axis troughs, and Cassegrain troughs (prior art). Cooper et al. [3] and Thesan S.p.a. [4] both employ a second degree of tracking freedom. In addition to single-axis trough tracking, the secondary optics are allowed to rotate or translate to compensate for the changing incidence angle (within the axial plane of the trough). The secondaries include hollow, rotating CPCs and solid dielectric reflectors. A design with an array of hollow CPCs operates near the prime focus. Alternatively, a laterally-translated cylinder or spherical lenses may be used [4].
FIG. 3 is a partial cross-sectional view depicting reflected and transmitted wavelength bands of a M2 spectrum splitter (prior art). In principle, such a design permits ultraviolet (UV) and infrared (IR) light to be collected as thermal energy at heat receiver 1 (HR1), and visible (Vis) and near infrared (NIR) to be collected as photovoltaic energy. A Cassegrain geometry poses a new obstacle to achieve high concentration. Since Cassegrain optics have a large focal length, the solar image formed below M2 is likewise larger. The primary concentration from a Cassegrain trough is thus lower than it would be at the prime focus. In order to keep the solar image small after the Cassegrain M2, the M2 size should be very large. However, this causes M2 to cast a large shadow on M1 (the trough). Alternatively, M2 can be made very small so that there is minimum shadowing effect. However, the size of the solar image at the base of the trough becomes very large. An alternative approach would require raising the receiver (photovoltaic cells) to be closer to M2. This allows some reduction in the focal length. However, this may affect trough stability due to a raised center of gravity. Therefore, without additional concentration, this design is impractical for both concentrated photovoltaic (CPV) and concentrated solar power (CSP) purposes.
U.S. Pat. No. 5,505,789 uses a tessellating line focus with solid secondary funnels to address the above-mentioned problems associated with Cassegrain optics [5]. U.S. Pat. No. 5,505,789 discloses line-focus lenses and a line-focused PV module. The whole system is an array of linear arched Fresnel lenses with a linear PV cell receiver located along the focal line of each lens. The photovoltaic cell receiver consists of high efficiency cells interconnected in a string with a solid secondary optical element adhesive bonded to the cells. The entrance aperture of each secondary optical element is rectangular in shape and the optical secondaries are butted up against each other in a line to form a continuous entrance aperture along the focal line. In addition to providing more concentrated sunlight, the solid optical secondaries shield the cells from air, moisture, and contaminants, and to a lesser extent against radiation damage. However, since this system does not employ Cassegrain optics or an additional means of concentrating light to the PV cells, it is a low concentrated CPV system. It is not obvious that this system can be modified to use Cassegrain optics, or that the light collected in such a system can be concentrated sufficiently for PV collection, in light of all the reasons mentioned above.
Other beam splitting approaches for solar power include Imenes et al. [6], dichroic filter designs for hybrid solar energy by DeSandre et al. [7], analysis of hybrid solar energy efficiencies by Hamdy et al. [8], and designs of hybrid solar systems by Soule et al. [9, 10].
FIG. 4 is a partial cross-sectional view of a Cassegrain hybrid trough system with PV at the bottom of the trough [11] (prior art). A similar Cassegrain trough system with beam splitter, but with no concentration at PV cells in a slit at vertex of trough is described by Jian et al. [12].
FIGS. 14A through 14C depict symmetric optics geometrically represented with isosceles triangles (prior art). Conventional symmetric optics must have a symmetric interference-free angular range of operation to maximize the capture of solar energy. Each optics section is able to converge edge rays, depicted in phantom in FIG. 14A, to a center point where they can be harvested. When incident light strikes the optical apertures at an angle of 0 degrees, as shown in FIG. 14A, there is no interference. However, when the optical elements are rotated about their individual axes, the element apertures eventually begin clipping the edge rays of the adjacent elements. The interference-free rotation limits are shown in each direction in FIGS. 14B and 14C. The limits are symmetrical about 0° incidence. Thus, when secondary tracking is achieved by individual rotation about (different) secondary tracking axes, interference between adjacent optical elements should be minimized. The interference envelope of each optical element is a function of the edge ray paths, dimension of the aperture, and angular range of motion.
It would be advantageous if a hybrid solar system using Cassegrain optics could be designed with optical elements tailored so that the capture of edge rays is asymmetric, resulting in an angular range which is also asymmetric, and with a range of motion able to match the range of solar incidence at any latitude.
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[3] “Theory and design of line-to-point focus solar concentrators with tracking secondary optics”, T. Cooper. G. Ambrosetti, A. Pedretti, and A. Steinfeld, Appl. Opt. vol. 52, 8586-8616 (2013).
[4] “Solar Receiver for a Solar Concentrator with a Linear Focus”, A. Balbo Divinadio and M. Palazzetti, Thesan S.p.a., US 2011/0023866, Published Feb. 3, 2011.
[5] “Line-focus photovoltaic module using solid optical secondaries for improved radiation resistance”, L. M. Fraas and M. J. Oneill, Entech Inc., U.S. Pat. No. 5,505,789, Granted Apr. 9, 1996.
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[13] Thomas Cooper, Gianluca Ambrosetti, Andrea Pedretti, and Aldo Steinfeld, “Theory and design of line-to-point focus solar concentrators with tracking secondary optics,” Appl. Opt. 52, 8586-8616 (2013).